Ring-A oxygenated derivatives of 5α- and 5β-cardenolides

Abstract

The introduction of an hydroxy-group α to the 3-keto-function of the 5α- and the 5β-cardenolides uzarigenone (4) and digitoxigenone (15) was carried out in three steps in good overall yield. The dimethyl acetals (5) and (16) of uzarigenone and digitoxigenone were pyrolysed to the respective Δ²- and Δ³-enol ethers (6) and (17). Epoxidation followed by spontaneous rearrangement led to respectively 2α-hydroxyuzarigenone (8) and 4β-hydroxydigitoxigenone (18). Sodium borohydride reduction of 2α-acetoxyuzarigenone (9) gave the 2- and 3-monoacetates (10) and (11) of gomphogenin (12)(2α-hydroxyuzarigenin), the latter monoacetate by acyl migration. On hydrolysis each formed gomphogenin (12). Reduction of the keto-function of 4β-acetoxydigitoxigenone (19) by lithium tri-t-butoxyaluminium hydride proceeded without rearrangement to yield the corresponding 3α-alcohol (20). The ¹³C n.m.r. data of 18 members of the title compounds are analysed.